Sound-wave generator



w. J. EISENBEIS ET AL ,177

SOUND-WAVE GENERATOR March 28, 1939.

Filed April 2'7, 1957 .HEATER CURRENT PLATE CURRENl I INVENTOR Z4 3 .22 Wm. J. Eisenbe/s8 34 E Chas. 0. Williamson Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE SOUND-WAVE GENERATOR Application April 27, 1937, Serial No. 139,254

4 Claims.

This invention relates to an apparatus, the principal function of which is the generation of sound Waves, although in certain applications, the generation of sound waves may be merely incidental to the attainment of a desired result.

Tuning-fork drives of various kinds have been known heretofore but have been characterized by certain features which made them unsuitable for some applications. Such drives with which We are familiar utilize a so-called mechanical or electromagnetic pick-up. As a result, they are not very accurate and are subject to the development of harmonics to an objectionable extent.

It is highly desirable, in certain fields, to have readily available a dependable source of a pure tone of constant frequency or pitch. As examples of such fields may be mentioned the tuning of musical instruments, the acoustic testing of auditoriums, and lecture demonstrations. As is well known, the tuning of musical instruments as now conducted involves the personal factor to a large extent resulting in inaccurate tuning. Our invention is particularly adapted to this problem since it provides a pure tone of accurate pitch to which various instruments may readily be tuned by observing the adjustment of the instrument at which the beats disappear. This eliminates the personal factor entirely. The invention is also useful in acoustic testing where a sound generator capable of producing a sustained pure tone is of great assistance.

In accordance with our invention, we provide a vibratile member such as a tuning fork with electromagnetic means for causing vibration thereof and energize the electromagnetic means, by suitable auxiliary equipment, so as to maintain the vibration thereof at its natural frequency. The auxiliary equipment includes a microphone subject to the sound emitted by the fork, and amplifying means controlled thereby for energizing the electromagnetic actuating means in synchronism with the vibrations of the fork.

A complete understanding of the invention may be readily gained from a consideration of the following detailed description thereof which is to be read in connection with the accompanying drawing illustrating a present preferred embodiment. In the drawing,

Fig. l is a plan view of the apparatus of our invention, the amplifying apparatus and electrical connections being omitted;

2 is a sectional view taken along the line II-II of Fig. 1 showing parts in elevation;

Fig. 3 is a similar view along the line III-III of Fig. 1;

Fig. 4 is an end view taken from the right of Fig. 1; and

Fig. 5 is a schematic diagram showing the amplifying apparatus and electrical connections.

The apparatus of our invention, aside from a two-stage electronic amplifier which is largely conventional, may conveniently be mounted on a base I0 and comprises a vibratile member, such as a tuning fork II, secured to a supporting bracket I2 mounted on the base. As shown, the fork has a stud I3 extending through the bracket and is secured thereto by a nut I4 threaded on the stud. A resonator box I5 tuned tothe same frequency as the fork II is also secured on the stud I3 by the nut I4, being spaced from the bracket I2 by a sleeve I6.

We provide electromagnetic means, indicated at IT, for actuating the fork I I. Such means take the form of a U-shaped magnet I8 having coils I9 thereon. This magnet may be a permanent magnet but any other convenient source of polarizing magnetic flux may be used instead. As clearly shown in the drawing, the magnet is disposed with its poles adjacent the prongs of the fork I I. The latter, of course, is of magnetic material, whereby energization of the coils at the proper frequency periodically varies the total flux emanating from the magnet poles I8 and vibrates the fork II. The magnet I8 may be supported on the base II] by any convenient means (not shown).

A resonator tube 2|] is mounted on the base I0 adjacent the fork II and resonator box I5. The resonator tube is also tuned to the natural frequency of the fork II. The tube is preferably mounted in soft rubber rings 2|, so that the vibration of the fork II will not be transmitted thereto mechanically through the base ID. A diaphragm ZZ is disposed adjacent one end of the tube 2!], and is preferably seated in a soft rubber mounting 23 for the reason just stated. A microphone 24, preferably of the carbon button type, is also attached to the mounting 23 and the diaphragm 22.

From the foregoing, it will be apparent that vibration of the fork II sets up sound waves which are reinforced by induced vibrations of the box I5. The sound waves emitted by the fork and box impinge on the diaphragm 22, causing it to vibrate at the some frequency as the fork II. The resonator tube 20 magnifies the effect of the sound waves on the diaphragm 22 and, while desirable for this reason, it may be dispensed with under certain conditions.

The tube 20 is subject at its open end to the pressure impulses created by vibration of the fork ll. At the open end of the tube, these impulses cause a maximum movement of the air particles with a minimum variation of pressure. The diaphragm 22 closes the other end of the tube and at that point, movement of the air particles is a minimum, but the pressure variation is a maximum. The microphone 24 is responsive principally to pressure variation. The tube 20 and diaphragm 22 thus provide a highly effective arrangement for concentrating the output of the fork upon the microphone.

As already intimated, the microphone 24 is connected to a two-stage electronic amplifier, the output of which is delivered to the magnet coils l9. Fig. 5 shows clearly the novel features of the amplifying circuit, although conventional features are indicated schematically.

Referring to Fig. 5, the microphone 24 is connected to the primary winding of an input transformer 25, in series with a control switch 26 and a current source 21. Amplification of the microphone current is obtained by means of a threeelectrode thermionic amplifier 28, the cathode of which is connected through a grid-bias battery 29 to a center tap on the secondary winding of the transformer 25. The grid of the amplifier 28 is connected to one end of the secondary of the transformer 25 through a high, variable resistor 30 and to the other end through a capacitance 3|. The anode-cathode circuit of the amplifier 28 includes the primary winding of an output transformer 32 and any convenient source of plate current. The secondary of the transformer 32 is connected in the input circuit of a second amplifier of conventional arrangement which is not illustrated in detail, but is indicated schematically at 33. The output of this second or power stage of amplification is delivered to the windings l9. An output transformer 34 connected to the amplifier may be conveniently used for this purpose.

The novel arrangement of the first amplifying stage is provided for the purpose of permiting an adjustment of the phase between the vibration of the fork II and the energization of the windings [9. It will be apparent that proper synchronism between these periodic operations must be obtained in order that the vibrations of the fork be reinforced by the periodic energization of the windings IS. The phase relation between fork vibration and energizaiton of the windings l9 may be varied substantially by reversing the polarity of the microphone current source 21. Adjustment of the resistance 30 will provide further adjustment so that substantially the entire range of 360 possible variation can be covered. Readjustment of the phase is necessary whenever the relative positions of the parts are changed.

The operation of the system will now be briefly described. With the arrangement of parts and electrical connections as shown in the drawing, the effect of the magnet I8 is to draw the prongs of the fork ll outwardly beyond their normal position. When it is desired to produce sound waves, the microphone switch 26 is closed. Current immediately begins to flow in the microphone circuit, and the windings I9 are energized by a current impulse resulting from the ampliflcation of the initial microphone current. This energization of the windings I 9 varies the flux emanating from the poles of the magnet II and sets the prongs of the fork II into vibration. Before this initial vibration ceases, the resonator box l5 emits sound waves by induced vibration and these waves impinge on the diaphragm 22. Vibration of the diaphragm causes pulsations in the microphone current in synchronism with the vibrations of the fork II. The pulsating microphone current is then amplified by the two-stage amplifier illustrated diagrammatically in Fig. 5, and the amplified current is delivered to the windings IS. The latter, being thus energized by an alternating current of a frequency the same as that of the fork ll, periodically vary the flux of the magnet l8 to reinforce the initial vibration of the fork. It will be understood that the polarities of the windings I9 and the magnet 18 are such that the operation described above will occur. When the windings I9 are de-energized, the magnet l8 draws the prongs of the fork outwardly past their neutral position.

The coils l9 are disposed at or near the nodal point of the first overtone of the fork, in order to minimize this undesired component. Its presence may be readily revealed, however, when desired for the purpose of demonstration, by placing the coils adjacent the tips of the prongs.

It will be apparent from the above description that the invention constitutes what might be designated an audio regenerator in that sound Waves of constant frequency are produced by a vibratile member, and a portion of the output thereof picked up acoustically, i. e., by pulsations created in the air, to reinforce the vibration of the member from which the sound waves originally emanated. The invention is characterized by numerous advantages which render it useful in many Ways. The resonator I5 not only has the effect of increasing the volume of the sound emitted by the fork l I, but also causes the output to build up very promptly on closure of the microphone switch 26. Without the resonator 15, the desired purity of tone would be produced but at a lower intensity, and the reinforcement of the fork vibration by amplification of the microphone current variations would take correspondingly longer. The amplification, of course, can be carried to any desired extent to produce whatever volume of sound may be desirable for a given application of the invention. For any degree of amplification, auxiliary control of the volume of the output may be had by means of an adjustable resistance 34 in series with the coils I 9. This provides a wide range of adjustment. The resonator tube 2!] serves to increase the effect of the vibrations of the fork and the reson- H ator box on the diaphragm 22. At the open end of the tube, amplitude variation is a maximum and at the closed end of the tube pressure variation is a maximum. Since the carbon button microphone is a pressure operated instrument, it is attached to the closed end of the tube, thus giving the most sensitive pick-up possible. and making possible instantaneous starting of vibrations. Since the diaphragm is supported resiliently on the tube, and the tube is likewise supported on the base It), the diaphragm responds solely to pulsations transmitted through the air and is unaffected by induced vibration of the base.

The magnet I8 is preferably strong enough variations in the voltage of the various current sources, ambient temperature, or the like.

The simplicity of the invention and its high degree of accuracy adapt it for numerous applications. It serves as an effective check upon the tuning of musical instruments, since the play ing of the latter in the presence of the apparatus While it is operating produces perceptible beats if the instrument is improperly tuned. Such pronounced beats are due to single frequency or pure tone of the output. These beats aiiord a much more effective check upon the accuracy of the tuning of an instrument than the mere comparison of the tone produced by one instrument with that or" another. In the latter case, even though the two instruments are played simultaneously, the beats which are created if they are not tuned to the same pitch, are less perceptible.

A further advantage of the invention is that there are no contacts engaging the fork prongs. Such contacts afiect the frequency of the fork. The drive for the fork is balanced, i. e., both prongs are simultaneously influenced to the same extent so there is no possibility of unbalance, and no tuning of the fork is required to cause it to vibrate at the correct frequency. The possibility of adjusting the phase relation between the vibration of the fork and the energization of the de-magnetizing windings 19 has already been mentioned.

In addition to the tuning of musical instruments, the invention is well adapted for the production of a sustained tone of constant frequency for other purposes, such as the acoustic testing of halls and auditoriums, demonstrations during lectures, and the like. The invention may also be used as a source of oscillating electric current by suitable auxiliary equipment which it is not necessary to describe. The Wave form of the acoustic output of the apparatus is almost perfect, being substantially free from harmonics or other distorting effects.

Although we have illustrated and described only the present preferred embodiment of the invention, it will be understood that changes in the apparatus and arrangement disclosed may be made without departing from the spirit of the invention or the scope of the appended claims.

For example, the pick-up may be electromagnetic and the drive for the fork acoustic, as described and illustrated in our article in The American Physics Teacher for May 1936, at page 93.

We claim:

1. In a sound-generating system, the combination with a resonator tube open at one end,a diaphragm closing the other end of the tube, a microphone connected to said diaphragm, and means mounting said tube efiective to prevent the transmission of vibrations thereto through media other than the atmosphere.

2. Apparatus for generating a sustained tone comprising a vibratile member having a predetermined natural frequency, electromagnetic means for causing vibration thereof, means for periodically energizing said electromagnetic means at said frequency and means for adjusting the phase relation between the energize.-

tion of said electromagnetic means and the vibrai tions of said member.

3. The combination with a vibratile member and electromagnetic means for causing vibration thereof, of means for energizing said means with current having the same frequency as the said member, said energizing means including a microphone, means mounting the microphone so that it is actuated solely by atmospheric pressure variations created by the vibrations of said member, a resonator tube adjacent said member, and a diaphragm closing one end of said tube, said microphone being secured to said diaphragm.

s. The combination with a vibratile member and electromagnetic means for causing vibration thereof, of means for energizing said means with current having the same frequency as the said member, said energizing means including a microphone, means mounting the microphone so that it is actuated solely by atmospheric pressure variations created by the vibrations of said member, a resonator tube adjacent said member, a diaphragm closing one end of said tube, said microphone being mounted on said diaphragm, and a mounting for said tube eifective to prevent the transmission of vibrations thereto except through the air.

WILLIAM J. EISENBEIS. CHARLES O. WILLIAMSON. 

